1. Field of the Invention
The present invention generally relates to methods and systems of fabricating devices. More particularly, this invention relates to methods for fabricating devices having embedded features such as optical and electrical feedthroughs as well as microfabricated features such as microchannels. Several embodiments of this invention relate to methods for fabricating microfluidic devices, having embedded features such as optical and electrical feedthroughs.
2. Background of the Invention
Often in many applications there is a need for transmitting optical or electrical signals from one side of a metallic part to the other. Often, such transmission needs to occur while there is a significant pressure difference between the two sides of the metallic part. Several solutions exist to this problem, including but not limited to the use of optical windows sealed with elastomeric o-rings or brazed to the metallic part and, respectively, use of electrical bulkheads, or feedthroughs, fabricated out of electrically insulating material (usually plastic) and incorporating one or several electrical pins which traverse the feedthrough.
In many applications there is also a need for a metallic part with very intricate details machined in it, such as small channels and holes, at length scales and resolutions down to the micron level, which are not easily achievable using conventional machining techniques. One example of a situation where such a need exists is in the manufacturing of metallic microfluidic devices.
There has been tremendous growth recently in the use of micromachining for fabricating microstructures, microsensors, and microfluidic devices, and in integrating these microstructures with electronic circuits. Micromachining is the process of forming structures having micron-sized detail by producing patterns on the surface or bulk of a substrate, or in layers of material deposited on a substrate. The material layers can be formed using a variety of processes, including sputtering, evaporation, physical vapor deposition, chemical vapor deposition and spin coating. Patterns are produced in these material layers by processes such as photolithography, precision physical machining, chemical etching, laser ablation, focused ion beam etching, ultrasonic drilling and electrodischarge machining, to yield the micromachined device.
Microfluidic devices, sensors, and systems, are also becoming increasingly common in several industries, such as pharmaceuticals, biotech, chemical engineering, homeland security, environmental engineering etc. For example, microfluidic devices can be used to transmit force and energy in hydraulic systems, such in the design of smart tools where the motion of the human hand must be scaled down to sub-millimeter dimensions, with a corresponding reduction in force. Further, microscale devices may also permit the assembly of a multiplicity of different functional devices in one compact, interconnected system. For example, individual microfluidic accessories such as mixers, micro-contactors, reactors, pumps, and valves may be added on a substrate containing microfluidic channels that connect such components in a microfluidic device
Microstructure technology offers distinct advantages over “macroscale” technology, including, for example, the ability to perform efficient and rapid chemical analyses at a lower cost per analysis, because of decreased sample volume requirements and increased throughput. Small sample volumes are advantageous because they allow a user to perform multiple analyses in parallel using a single sample on a single chip.
A variety of microfluidic applications require electrical conductors. Conductors can be used to form electrical interconnections (“interconnects”) between elements of a microfluidic device, such as electrodes, and elements external to the device, such as power sources and data acquisition systems. Such interconnects can provide electrical flow to the electrode to power electrostatic, electromagnetic or electrohydrodynamic micropumps, to operate microvalves, or to induce electrolysis of a sample fluid. Conductors may also be used to guide electrical signals from sensor located on the microfluidic chip to data acquisition and analysis systems that are external. Conductors can also be used as resistive heaters for sample fluids or as temperature sensors in microfluidic applications.
Developments in the semiconductor processing industry have facilitated the fabrication of micron-sized structures, including sensors and monitoring systems that can be used in microfluidic devices. The fabrication of microfluidic devices requires a method of producing fluidic connections, referred to as microchannels, and electrical interconnects between regions of a single device or between a device and accessories such as power supplies, data acquisition systems, automatic valves, pumps, or syringes. Microfluidic devices typically consist of several components, such as: microchannels for fluid flow (typical dimensions may include 100 micron square cross section and a few centimeters in length); fluidic inlet and outlet ports, with appropriate fittings (typically able to accommodate capillary tubing); fluidic manipulation components (microvalves, micropumps, droplet or particle manipulators etc.); and embedded sensors and measurement devices (such as pressure, optical, chemical and electrical sensors etc.).
The fabrication of microfluidic devices can involve fabricating by one of several available technologies (below), open-top microchannels on a plane substrate, and then bonding the resulting part to a plane or structured layer, henceforth called the top layer, that provides a fourth wall to the channels, completely enclosing them. Inlet and outlet ports typically consist of through-holes that connect an end of one channel to some kind of tubing fitting. The substrate or the top layer may include various additional components of the system, such as sensors, optical fibers, optical windows, electrodes, valves and pumps, etc.
An existing fabrication technology for creating microchannels can include cure-molding of elastomers, e.g., soft lithography (J. C. McDonald et al., “Fabrication of microfluidic systems in poly(dimethylsiloxane)”, Electrophoresis, 21, p. 27 (2000), to replicate photoresist master molds. The resulting surface with molded open-faced channels can typically be bonded to a glass layer or to another elastomer layer after an oxygen plasma treatment. Typically, this method results in soft devices that are unable to withstand high pressures, and are quite susceptible to swelling in the presence of organic solvents. The attractiveness of this technique consists of the fast prototyping capabilities, and optical clarity of the silicone elastomers typically used.
Another known fabrication technology for creating microchannels may include injection molding or embossing of plastics (typically low temperature materials such as polystyrene, polycarbonate, polymethylmethacrylate, polypropylene, cyclic olefin copolymer, but also including high-performance materials such as PEEK). Thermal lamination, adhesive bonding or plasma treatments in this case can be used for subsequent bonding. Very few plastics provide a strength/chemical compatibility/temperature resistance combination that is attractive for use in harsh environments, one of these being PEEK. N2 plasma proves to be particularly useful in creating a strong PEEK to PEEK bond, as recently reported, often stronger than the intrinsic material strength (H. Mühlberger, A. E. Guber, W. Hoffmann—“Microfluidic Polyether Ether Keton (Peek) Chips Combined With Contactless Conductivity Detection For mTAS” Proceedings of the MicroTAS Conference 2005, p. 184 (2005)). One issue among many other potential issues limiting the use is the fabrication of the mold insert tool, which needs to be made of a very strong material (such as Nickel in pure form or in one of several alloys) but at the same time have very fine microstructures machined into it. Still, another issue is the optical opacity of some plastic materials (including PEEK), requiring complicated post-fabrication processing to incorporate optical pathways (e.g. by embedding or adhesively bonding optical fibers).
Other known fabrication technologies for creating microchannels include co-firing of ceramic materials. This method has potential of providing microfluidic devices which provide for some temperature and pressure qualification of the materials and bonds (K. D. Patel, K. W. Hukari, K. A. Peterson—“Cofired Ceramic Microdevices For High Temperature And High Pressure Applications” Proceedings of the MicroTAS Conference 2005, p. 709 (2005)).
Another known fabrication technology for creating microchannels may include silicon micromachining. Silicon micromachining can be used to etch different channel geometries in single-crystal silicon wafers, the resulting channels being subsequently sealed with one or several layers of borosilicate glass (e.g. Pyrex) by anodic bonding. This method has many drawbacks such as the brittleness and chemical resistance of the materials, which ultimately limit its use in the field. The bond strength of the interface may also be an issue if high internal pressures are required (A. Hanneborg, M. Nese, P. Ohlckers—“Silicon-to-Silicon Anodic Bonding With a Borosilicate Glass Layer” J. Micromech. Microeng. 1, p. 139 (1991)). Bonding between two silicon wafers can also be achieved using an intermediate glass layer, direct (or fusion) bonding and eutactic bonding. Other known fabrication technologies for creating microchannels include can glass etching (usually of chemical nature, or by powder blasting or ultrasonic machining) can be used to create channels in glass, which can then be bonded to another layer of glass by direct bonding. A downside of this method is the difficulty to create very high aspect ratio, vertical wall structures.
Another known fabrication technology for bonding of metallic external components (such as inlet and outlet tubes) to the materials such as Nickel parts, can be achieved by diffusion bonding (or solid-state welding), welding or brazing. Several companies exist which specialize in this kind of operations, and reports also exist in the scientific literature (T. R. Christenson, D. T. Schmale—“A Batch Wafer Scale LIGA Assembly and Packaging Technique via Diffusion Bonding,” in Proc. IEEE Int. Conf. MEMS, p. 476 (1999)).
Another known fabrication technology for creating microchannels may include electrolytic metal deposition. Electrolytic metal deposition can be used to grow layers of metal however a substantial drawback in using such a method is that the metal grows only on conductive surfaces, so metallization is required in order to create a perfect metal topographical copy of a surface. Metallic parts fabricated using the LIGA technology may be used as metallic parts by themselves or as tools for plastic injection molding inserts (LIGA—W. Bacher, K. Bade, B. Matthis, M. Saumer, R. Schwarz—“Fabrication of LIGA mold inserts” Microsystem Technologies 4, p. 117 (1998)). This method has been used to create enclosed channels and chambers, typically by using sacrificial layers, or by bonding together several thin layers of electroplated metal using diffusion bonding techniques (T. R. Christenson, D. T. Schmale “A Batch Wafer Scale LIGA Assembly and Packaging Technique via Diffusion Bonding,” in Proc. IEEE Int. Conf. MEMS, p. 476 (1999)). A drawback of the technique is the expensive and time-consuming electrolytic metal deposition step, often requiring several weeks of deposition to create a metallic part a few mm thick.
Other known fabrication technologies for creating microchannels may include other precision engineering machining methods, such as micro electro discharge machining, laser ablation, punching/drilling/embossing, wet etching of intermediate metal foils etc. (W. Ehrfeld, V. Hessel, H. Lowe—“Microreactors—New Technology for Modern Chamistry” Wiley-VCH Verlag, Weinheim, Germany, pp. 15-40 (2000)). These typically impose resolution limitations when compared to photolithography-type methods, and tool size may affect the minimum feature size and/or density of microstructures. Precision machining provides, however, a viable way to creating molding inserts for microchannel structures, particularly when they are not too densely packed (Mikell Knights, “Micro Molds Make Micro Parts”, PlasticsTechnology online: see plasticstechnology.com/articles/200212fa1.html).
Chemical Vapor Deposition (CVD) is a thin film deposition method relying on chemical reactions between gaseous precursors occurring on or near the substrate surface. CVD is usually performed in a CVD furnace at very high temperatures, which many materials cannot tolerate. CVD should be therefore be performed as one of the first steps in the silicon fabrication process. A variant of the CVD commonly used in the microelectronics industry is plasma-enhanced CVD (PE-CVD) whereby a plasma of the reacting gases is created to enhance reactivity and allows faster deposition rates at lower temperatures. Several materials can be deposited by CVD or PE-CVD, most commonly: polysilicon, silicon nitride, silicon oxide, and some metals. The resulting films are usually conformal; their properties depending a lot on process parameters: temperature, pressure, gas flow rates. A CVD variant called metalorganic CVD or MO-CVD can be used for deposition of thin as well as very thick conformal metal layers, such as CVD metal fabrication (see Terekhov, D. S., O'Meara, M.: “Recycling metals using the MO-CVD process”, Proceedings of the TMS Fall Extraction and Processing Conference, p. 487 (2000).
Accordingly, there is a need for improved methods and systems capable of providing devices worthy of use in demanding environments so as to withstand, by non-limiting example, high pressures, high temperatures and harsh environments. There is also the need for versatile fabrication methods capable of batch manufacturing and/or batch processing of parts having characteristics, such as: high structural strength and excellent chemical resistance.